Technical Abstract:
This study evaluated changes in plasma amino acids in late-gestating (beginning 58 ± 1.02 d prior to calving), primiparous, winter-grazing range heifers receiving a wheat middling based supplement without (CON) or with rumen-protected methionine (MET). Plasma was collected on d -2 and d 0 (start of MET supplementation just prior to individually receiving supplement at 0700 h). Plasma was sampled again on d 40, 42 and 44 prior to supplementation at 0700 h and 1100 h (4 h after receiving daily supplement). Data were analyzed with cow as the experimental unit. Continuous variables were analyzed by the main effects of treatment, date, or time and their interaction when appropriate. Comparable BW (P = 0.74) and BCS (P = 0.65) over the 44-d metabolism trial were found between both CON- and MET-fed heifers. MET-supplemented heifers had greater (P < 0.0001) plasma concentrations of methionine indicating that the rumen-protected technology successfully presented methionine to the small intestine for absorption. Notable decreases in plasma AA concentrations in MET-fed heifers after 44 d included leucine (P = 0.04), valine (P = 0.03), and serine (P = 0.05). Glycine, the most abundant amino acid in maternal blood, was greater (P = 0.05) in CON-fed heifers than MET-fed heifers. Lower glycine concentrations in MET heifers were due to the role of glycine in catabolism of methionine. Branched chain amino acids (BCAA) are instrumental in metabolism of maternal protein and there were several significant differences in BCAA between CON and MET fed heifers. Isoleucine, leucine and valine resulted in notable decreasing percent changes from d 0 to 44 (P = 0.06, P = 0.04, P = 0.03, respectively) for MET-fed heifers. These results imply methionine is a limiting amino acid in late-gestating heifers grazing dormant range forages due to improved utilization of other amino acids when supplemental methionine was provided.